An interlaced video is a succession of approximately 50 or 60 fields per second with each of the fields carrying only the alternating rows displayed in each frame of the video. Interlaced video is often derived from motion picture film materials photographed in a 24 or 30 frames-per-second progressive manner for display on cathode ray tube (CRT) type of displays. Interlacing is generally performed to achieve bandwidth reduction, but results in a reducing picture display quality. However, because the reduction in picture display quality from interlacing is not very noticeable on CRT displays, interlacing has been used as an elementary compression technique for CRT displays.
As most of today's digital television displays require progressive video inputs, all video materials in the interlaced format need to be converted to a progressive video format in order to be properly displayed on the digital TV displays. This process is referred to as deinterlacing. One typical deinterlacing technique is known as field combination (or weaving) which simply combines the appropriate fields together to restore the original images.
To weave the correct fields together, the deinterlacer needs to detect cadence of the interlaced video. Cadence generally refers to a pattern of successive fields that correspond to the frames of the original video (i.e., conversion pattern). If the cadence cannot be properly detected, the deinterlacer will not be able to properly weave the correct fields together and may discard video data erroneously.
The detection of cadence may be complex and difficult. For example, motion picture film is typically progressive and is based on 24 frame-per-second sequence while the NTSC format for TV broadcast is 60 fields per second. To convert a motion picture film into an interlaced video in NTSC format, a 3:2 pull-down repeating cadence is used to generate three fields from one film frame and two fields from the next film frame. In addition, sometimes every twelfth field is dropped to accelerate the film and fit the film within a given time slot. This loss results in a 3:2:3:2:2 repeating cadence.
Moreover, although the 3:2 repeating cadence is the most common format, other repeating cadences (e.g., 2:2, 2:3:3:2, and 3:3) may also be used to interlace the progressive original film. It is also possible that an interlaced video may have one portion of the video interlaced with a 3:2 cadence while having another portion of the interlaced video converted with a different cadence (e.g., 2:2 or 3:3). It is also possible that an interlaced video does not have any cadence. Furthermore, it is also possible that a field of an interlaced video contains both cadenced content and un-cadenced content. For example, in the case of a foreign film where subtitles (un-cadenced content) are overlaid on top of the original film content (cadenced content).